1. Field of the Invention
This invention relates to a process and system for separation of hydrogen sulfide (H2S), carbon dioxide (CO2), and hydrogen (H2) from a fluid mixture comprising said hydrogen sulfide, carbon dioxide, and hydrogen. More particularly, this invention relates to a membrane reactor process and system that combines hydrogen sulfide removal, water-gas-shift reaction, hydrogen separation and carbon dioxide separation within a single system and, more particularly, within a single membrane configuration. The process and system of this invention is suitable for use in processing synthesis gas generated from gasification and reforming of carbonaceous materials for hydrogen production and carbon dioxide capture. After removal of hydrogen sulfide by the use of a front-end membrane section, the CO conversion of the water-gas-shift reaction from the synthesis gas stream is enhanced by the complementary use of two additional membranes to separate the carbon dioxide and hydrogen. By virtue of this invention, hydrogen production efficiency is increased, using a system having a smaller physical footprint and lower capital costs than can be achieved using conventional systems.
2. Description of Related Art
A conventional process for production of hydrogen is shown in FIG. 1. As shown therein, hydrogen can be produced in a reactor 10 from carbonaceous materials such as coal, biomass and petroleum coke by reaction with oxygen and steam in a gasification reactor under elevated temperatures, about 800-2000° C., or by natural gas reforming in which steam is reacted with natural gas in a catalytic reactor. The generated product gas from either gasification or reforming, referred to as synthesis gas or syngas, comprising mostly hydrogen and carbon monoxide, is then sent to a shift reactor 12 to convert the carbon monoxide to hydrogen by the water-gas-shift reaction. However, before it can be processed by the downstream shift reactor, the syngas from the gasification process requires gas cleanup in a gas cleaning system 11 to remove contaminants, in particular, sulfur compounds. After the shift reactor, the hydrogen is separated and purified in a pressure swing adsorption (PSA) unit 14 or by other similar hydrogen separation techniques. If necessary, carbon dioxide can be removed in a CO2 removal unit 13 upstream of the PSA unit 14 to obtain a CO2-enriched stream and increase the hydrogen recovery in the PSA unit.
A number of processes for the recovery or removal of a variety of gaseous mixture components from gas streams are known to those skilled in the art. Although the processes vary widely, they generally involve some form of solvent absorption, adsorption on a porous adsorbent, distillation, or diffusion through a semipermeable membrane.
Membranes are thin barriers that allow preferential passage of certain components of a multi-component gas mixture. Most membranes can be separated into two types: porous and nonporous. Porous membranes separate gases based on molecular size and/or differential adsorption by small pores in the membrane. Gas separation membranes used in natural gas applications are often nonporous or asymmetric and separate gases based on solubility and diffusivity. In a typical membrane separation process, a gas is introduced into the feed side of a module or vessel that is separated into two compartments by the permeable membrane. The gas stream flows along the surface of the membrane and the more permeable components of the gas pass through the membrane barrier at a higher rate than those components of lower permeability. After contacting the membrane, the depleted feed gas residue stream, known as the retentate, is removed from contact with the membrane by some suitable means. The gas on the other side of the membrane, the permeate, is removed from contact with the membrane through a separate means. By the term “permeate” is meant that portion of a feed stream which is withdrawn at the second side of the membrane, exclusive of other fluids such as sweep gas or liquid which may be present on the second side of the membrane.
U.S. Pat. No. 6,579,331 B1 teaches a CO2-selective membrane process that is useful for the purification and/or water-gas-shift reaction of a reformed gas. The purification process selectively removes CO2 from the reformed product, thereby enriching the reformed product in H2 and increasing the H2/CO2 ratio. The CO2 selective membrane is a composition comprising a hydrophilic polymer and at least one ammonium halide salt. Suitable hydrophilic polymers are indicated to include polyvinylalcohol, polyvinylpyrrolidone, polyethyleneoxide, polypropyleneoxide, polyacrylamide, polyvinylacetate and blends and copolymers thereof.
U.S. Pat. No. 6,860,920 B2 teaches gas separation membranes formed from polyester-polyether block copolymers which are useful for separating gases, especially polar gases from mixtures that contain both polar and non-polar components. In particular, the membranes are indicated to be suitable for removal of gases including CO2, H2S, and SO2 from mixtures comprising non-polar gases such as helium, hydrogen, nitrogen or hydrocarbons.
U.S. Pat. No. 5,451,386 teaches a hydrogen-selective membrane comprising a tubular porous ceramic-support having a palladium metal layer deposited on an inside surface of the ceramic support. The membrane is suitable for separating hydrogen from feed gas mixtures comprising, in addition to hydrogen (H2), carbon monoxide (CO), nitrogen (N2), water (H2O), hydrogen sulfide (H2S) and ammonium (NH3). See also U.S. Pat. No. 4,810,485, which teaches a hydrogen forming process and apparatus wherein one side of a hydrogen ion porous and molecular gas nonporous metallic foil is contacted with mixed gases comprising molecular hydrogen formed by a chemical reaction in a hydrogen production zone and the molecular hydrogen is dissociated and passed as ionic hydrogen to the other side of the metallic foil from which it is withdrawn; U.S. Pat. No. 5,229,102, which teaches a diffusion process from steam reforming of a hydrocarbon to produce H2, CO and CO2 in which a generally tubular, porous, ceramic membrane, having a catalytically active metallic substance, disposed in a heated reaction zone in a container is contacted on one side with a hydrocarbon-and steam-containing fluid stream to produce CO, CO2, and H2 and is exposed to a second fluid stream on its opposite side in such a manner so as to promote hydrogen diffusion through the membrane; U.S. Pat. No. 6,207,132 B1 which teaches an apparatus for producing hydrogen which includes a reforming chamber containing a reforming catalyst bed for reforming a carbonaceous fuel into hydrogen, at least one hydrogen-permeable membrane tube disposed inside the reforming chamber surrounded by the reforming catalyst bed and confining therein a hydrogen compartment, and at least one oxidation chamber adjacent to the reforming catalyst bed for burning gas not permeable to the membrane tube and for providing heat to the reforming chamber; U.S. Pat. No. 6,221,117 B1, which teaches a fuel processing system comprising a steam reformer which produces hydrogen from a feedstock consisting of water and alcohol and/or hydrocarbon feedstock, a reforming catalyst, and a hydrogen-selective membrane module; and U.S. Pat. No. 6,231,831 B1, which teaches a hydrogen separation membrane comprising palladium or palladium alloy.
It will be appreciated by those skilled in the art engaged in the study of membrane-gas separation processes and systems that gas separation membranes exist in a variety of forms depending upon the gas(es) to be removed and that the conditions under which the membranes operate vary widely, also dependent upon the gas(es) to be removed. For the removal of H2S, CO2 and H2 from a gaseous mixture comprising these components, as evidenced herein above, there exist a variety of membranes and processes suitable for this purpose. However, no single process and system have been identified that are suitable for removal of all three of these components from a gaseous fluid mixture of which they are components.